This letter studies the effect of AlGaN back barriers in the dc and RF performance of In0.17Al0.83N/GaN high-electron mobility transistors grown on SiC substrates. When compared to conventional structures without a back barrier, the back bar- rier effectively prevents the degradation of drain-induced bar- rier lowering and significantly improves the output resistance in sub-100-nm-gate-length devices. The reduction in short-channel effects helps to increase the frequency performance of AlGaN back-barrier devices. For a 65-nm gate length, the current gain cutoff frequency (fT ) of a transistor with an AlGaN back barrier is 210 GHz, which is higher than that of the standard device with the same gate length (fT = 195 GHz). Index Terms—AlGaN back barrier, current gain cutoff fre- quency (fT ), drain-induced barrier lowering (DIBL), GaN, high- electron-mobility transistor (HEMT), InAlN, output resistance.